A continuously variable transmission (“CVT”) typically incorporates:—                (a) a variator—a device having a rotary input and a rotary output which is able to vary the ratio of its input speed to its output speed (the “variator ratio”) in a stepless manner, and        (b) associated gearing by means of which the variator is coupled between a rotary power source, such as an engine, and a point of power usage, for example the driven wheels of a motor vehicle.        
The overall speed ratio provided by the transmission as a whole (the “transmission ratio”) is a function of the variator ratio, but generally not identical to it, being modified by the associated gearing.
It is well known to incorporate in the gearing a “shunt” gear arrangement, typically of epicyclic type. Shunt gears can serve to recirculate power, reducing the power handled by the variator itself, and to provide a condition known in the art as “geared neutral”. The shunt typically has two rotary inputs coupled to opposite sides of the variator, and a rotary output coupled e.g. to the final gearing and so to the vehicle wheels. At a certain variator ratio (the “geared neutral ratio”) the two inputs to the shunt cancel each other out, leaving the output stationary. This condition is referred to as “geared neutral” and enables the transmission output to be brought to a halt without it being physically de-coupled from the moving engine. Such a transmission can thus be used without any “starting device”, such as the manual clutch or torque converter of a conventional automotive transmission, used to couple/decouple engine and transmission upon vehicle launch and upon braking of the vehicle to rest. Variator ratios to one side of the geared neutral ratio provide reverse output rotation and reverse vehicle travel. Variator ratios to the other side of the geared neutral ratio provide forward output rotation and forward vehicle travel. When the variator is at the geared neutral ratio the driven wheels and the vehicle are at a halt.
Typically the gearing of a CVT incorporates one or more clutch devices, engagement/disengagement of which allows the transmission to switch between “regimes”. Transmission ratio is a function of variator ratio, but in each regime the relationship between variator ratio and transmission ratio is different. For example, motor car transmissions are often designed to provide two regimes—high and low. Low regime provides reverse, geared neutral and low forward gears. High provides higher forward gears.
Ratios are selected in the gearing such that when the variator reaches a certain variator ratio (the “synchronous ratio”) close to one end of its range, a change from low to high regime causes no change in transmission ratio. A regime change at synchronous ratio can be made smoothly, without large discontinuity in the torque at the vehicle wheels or change of engine speed.
The use of multiple regimes is desirable with regard to the transmission's energy efficiency. The variator itself is typically the least efficient part of the transmission. In any given regime, if the spread of ratios provided by the transmission as a whole is greater than the ratio spread of the variator, then the shunt is “power split”. That is, only part of the total power is transmitted through the variator. Reducing the ratio spread in a given regime reduces the proportion of the total power through the variator, and so can improve efficiency and reduce the necessary dimensions and specification of the variator itself. For such reasons it can in some cases be desirable to provide more than two regimes. Large road-going trucks provide one example. Energy efficiency is an important consideration for such vehicles and their engines create particularly high power and torque, the handling of which by the variator could be problematic in a two regime transmission.
A known example of a CVT operable in three or more regimes is provided in published international patent application WO 94/24462, in the name of Torotrak (Development) Limited. Its United States counterpart is U.S. Pat. No. 5,643,121. The transmission in question uses two epicyclic shunt gears. One of these is referred to in that document as the “power splitting” epicyclic because it receives power from the engine and splits it between first and second shafts, accommodate changes in their relative speeds. The variator itself has its input connected to the first shaft and its output connected to the second shaft, so (for a fixed engine speed) an increase in variator ratio causes the second shaft to speed up and the first shaft to slow down, whilst a decrease in variator ratio causes the second shaft to slow down and the first to speed up. Each shaft is able to be selectively coupled to the vehicle wheels via at least one clutch/gear arrangement. Consider what happens as transmission ratio is increased. Initially, say, the first shaft is connected to the wheels via a first clutch/gear arrangement. The second shaft is disconnected and so freewheels. The variator is swept through its ratio range to increase the speed of the first shaft and the speed of the driven wheels. Eventually the variator reaches the end of its ratio range and a synchronous regime change is initiated, disconnecting the first shaft and connecting the second shaft to the wheels through a second clutch/gear arrangement. At this point the direction of change of the variator ratio is reversed. The variator is then swept back through its ratio range, increasing the speed of the second shaft and of the driven wheels. When it reaches the opposite end of its ratio range, a change to a still higher regime can be made by disconnecting the second shaft and connecting the first shaft to the wheels through a third clutch/gear arrangement. In principle, by providing each shaft with multiple clutch/gear arrangements for driving the wheels at different ratios, any number of regimes can be provided.
The second of the shunt gears is referred to in WO 94/24462 as the “power recirculating” epicyclic and serves to provide a low regime, containing geared neutral. Its inputs are connected across the variator and its output is connectable via a clutch to the driven wheels. In low regime, the first and second shafts are both disconnected from the wheels which are instead driven by the output of the power recirculating epicyclic.
The design and layout of such a transmission is problematic. WO 94/24462 shows arrangements in which the two epicyclic shunts are co-axial with each other and with the transmission input, but the variator itself is laterally offset from their common axis. This is not always convenient from a packaging point of view, nor does it necessarily allow the number of gears in the power transfer paths to be minimized, and additional gearing is undesirable as it increases transmission energy dissipation.
A transmission which operates on essentially similar principles but has a different layout is disclosed in published international patent application WO94/16244, again in the name of Torotrak (Development) Limited. Its US counterpart is U.S. Pat. No. 5,564,998. In that transmission the two epicyclic gear arrangements are co-axial with, and on either side of, the variator, but the transmission's output is made through a lay shaft which is offset from the variator's axis, and this too can be problematic with regard to packaging.
The designations “input” and “output” have been used above and will be used repeatedly below with reference to the shafts or other rotary members through which components such as the variator—and indeed the transmission itself—couple to other components. It is a useful nomenclature to distinguish one side of the component from the other, but it should be understood that in most cases the designation is essentially arbitrary, and that the flow of power (while it is assumed to be from engine to wheels in the discussion below) will not necessarily always be from input to output.